The invention relates to correction of DC-offset caused by an I/Q modulator in a transmitter.
In new radio systems, the scarcity of radio frequencies makes it necessary to use spectrum-efficient modulation methods. In Europe, a new radio system standard has been developed for PMR (Professional Mobile Radio) users, called TETRA (Terrestrial Trunked Radio). π/4-DQPSK (π/4-shifted Differential Quadrature Phase Shift Keying) has been selected as the modulation method of the system. As far as a transmitter is concerned, a drawback to the modulation method is the variation in the amplitude of the envelope of a radio frequency signal, which causes InterModulation (IM) in a nonlinear amplifier. The IM results spread the spectrum of a transmitted signal and thus tend to reduce the gain obtained from using the linear modulation method. The IM results cannot usually be filtered since they are formed extremely close to the desired signal. With constant-amplitude modulation methods, no spreading of the spectrum occurs; therefore, the signal can be amplified by a nonlinear amplifier.
A trunked PMR system, wherein different user groups share the same radio channels, has stringent requirements regarding adjacent channel interference caused by a transmitter. These requirements necessitate good linearity in the transmitter of the radio system used.
In a power amplifier, good linearity is only achieved with poor efficiency. However, the efficiency of portable equipment should be as high as possible for the operation time to be sufficient and in order not to waste battery capacity. In addition, at least relatively good efficiency is required of power amplifiers at base stations in order to avoid cooling problems. Sufficient efficiency and linearity can only be achieved by linearizing the transmitter.
If the nonlinearities of an amplifier were known in advance, it would be possible to form inverse functions of the nonlinearities to convert the input signal, whereby the nonlinearities would be cancelled. The characteristics of the amplifier do not, however, stay the same but they change due to, for example, aging, warming up, and according to the radio channel and power level used. In addition, amplifiers have individual differences. Linearization methods are needed that are capable of adjusting adaptively to changing conditions. Research has been conducted on many different linearization methods, and three have been found to possess characteristics suitable for practical radio systems. These methods are feedforward, Cartesian feedback and predistortion. A linearization method can also be adaptive.
Thus, if the nonlinear transfer function of the amplifier is known and if it does not vary as a function of time, the signal to be transmitted can be linearized by applying to the signal a suitable transfer function causing predistortion. Hence, the signal outputted from the amplifier can be made linear. This method is called predistortion. At baseband, for example, predistortion can be carried out by using a lookup table (LUT) into which are stored conversion parameters causing predistortion, i.e. predistortion parameters, in which case the conversion parameters to be used at a given time are selected on the basis of the amplitude of the signal to be predistorted.
In a signal measured from the feedback branch of a predistortion system, a direct current component of a varying magnitude, i.e. DC-offset, occurs owing to the carrier wave leakage of an I/Q modulator in the forward branch. An I/Q modulator operates on a quadrature modulation basis. It enables two independent signals to be combined in the transmitter and be transmitted on the same transmission band and the signals to be separated again at the receiver. The principle of quadrature modulation is that two separate signals, I and Q (Inphase and Quadrature phase), are modulated by using the same carrier wave frequency, but the phases of the carrier waves differ from each other in that the carrier wave of signal Q lags 90° the carrier wave of signal 1. After modulation, the signals are summed. Thanks to the phase difference, the I- and Q-signals can be separated from each other when the sum signal is demodulated. Due to the DC-component, it is difficult to measure and compensate for the nonlinearity of the amplifier; therefore, the DC-component is to be removed. Correction circuits with fixed correction parameters set therein have been used in connection with predistortion systems for correcting the DC-offset caused by the I/Q modulator. The problem is then manufacturability since each I/Q modulator requires unique parameters which must be set as early as during the production. Furthermore, the carrier wave leakage does not stay constant but creeps with time. Consequently, the fixedly-set correction does not work properly in the long run. Another known solution has been disclosed in “New Methods for Adaptation of Quadrature Modulators and Demodulators in Amplifier Linearization Circuits” by Cavers, J. K. in IEEE Transactions on Vehicular Technology, Vol. 46, No. 3, August 1997, pp. 707 to 716. It discloses a system wherein the correction parameters are determined by comparing the output of an I/Q modulator with the input thereof. The solution requires, however, an additional feedback branch independent of the feedback branch of the predistorter, which results in a complex arrangement.